1. Field of the Invention
The present invention relates to a polarized illumination optical system adapted to be used for example with a projection display apparatus employing a liquid crystal light valve or the like.
2. Description of the Prior Art
In the case of a liquid crystal projector, for example, it has been necessary to use an illumination optical system for supplying a linealy polarized illuminating light to its liquid crystal light valve. Then, heretofore known illumination systems have been so designed that after the light from a light source has been converged to a parallel beam, the parallel beam is separated into an S-polarized light and a P-polarized light by a polarizing beam splitter (hereinafter referred to as a PBS) so that the S-polarized light is utilized as an illumination light, whereas the P-polarized light is reflected by reflecting means and a part of the reflected light is converted to an S-polarized light by for example a quarter-wave plate or phase plate thereby utilizing it as an illumination light.
As one of such conventional illumination optical systems, a method has been known in which as for example, the reflected light is again reflected on the light source side and an S-polarized light component is taken out from the rereflected light so as to be utilized as an illumination light. In this case, the rereflection takes the form of irregular reflection and thus the polarizing condition of the rereflected light is disturbed thereby giving rise to the disadvantage that the polarized light components of the rereflected light cannot be fully utilized. Also, this method is disadvantageous in that not only the reflected light is considerably absorbed by the reflecting part on the light source side thus causing a loss in the light quantity, but also the reflected light is subjected to reflection a plurality of times so that even if the rereflected light is transmitted through a phase plate or the like, it is difficult to convert practically all of its components into an S-polarized light thus also increasing light quantity loss.
There has been known another conventional method in which of the polarized light components separated by a PBS, the S-polarized light is utilized as a first illumination component and the other polarized-light component or the P-polarized light is reflected in the same direction as the first illumination component and immediately passed through a phase plate thereby utilizing its S-polarized light component as a second illumination component. In this case, since the two illumination components are taken out separately, there are disadvantages that the light quantity cannot be utilized effectively on the surface to be illuminated and illumination irregurality tends to be caused and that it is necessary to provide some distance between the optical system and the surface to be illuminated.